Subject: [PATCH AUTOSEL 4.19 34/84] bpf, mips: Limit to 33 tail calls

From: Sasha Levin <[email-masked]>
Date: Fri, 27 Dec 2019 12:43:02 -0500

From: Paul Chaignon <[email-masked]>

[ Upstream commit e49e6f6db04e915dccb494ae10fa14888fea6f89 ]

All BPF JIT compilers except RISC-V's and MIPS' enforce a 33-tail calls
limit at runtime.  In addition, a test was recently added, in tailcalls2,
to check this limit.

This patch updates the tail call limit in MIPS' JIT compiler to allow
33 tail calls.

Fixes: b6bd53f9c4e8 ("MIPS: Add missing file for eBPF JIT.")
Reported-by: Mahshid Khezri <[email-masked]>
Signed-off-by: Paul Chaignon <[email-masked]>
Signed-off-by: Daniel Borkmann <[email-masked]>
Acked-by: Martin KaFai Lau <[email-masked]>
Link: https://lore.kernel.org/bpf/b8eb2[email-masked]
Signed-off-by: Sasha Levin <[email-masked]>
---
 arch/mips/net/ebpf_jit.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/mips/net/ebpf_jit.c b/arch/mips/net/ebpf_jit.c
index 9bda82ed75eb..3832c4628608 100644
--- a/arch/mips/net/ebpf_jit.c
+++ b/arch/mips/net/ebpf_jit.c
@@ -586,6 +586,7 @@ static void emit_const_to_reg(struct jit_ctx *ctx, int dst, u64 value)
 static int emit_bpf_tail_call(struct jit_ctx *ctx, int this_idx)
 {
 	int off, b_off;
+	int tcc_reg;
 
 	ctx->flags |= EBPF_SEEN_TC;
 	/*
@@ -598,14 +599,14 @@ static int emit_bpf_tail_call(struct jit_ctx *ctx, int this_idx)
 	b_off = b_imm(this_idx + 1, ctx);
 	emit_instr(ctx, bne, MIPS_R_AT, MIPS_R_ZERO, b_off);
 	/*
-	 * if (--TCC < 0)
+	 * if (TCC-- < 0)
 	 *     goto out;
 	 */
 	/* Delay slot */
-	emit_instr(ctx, daddiu, MIPS_R_T5,
-		   (ctx->flags & EBPF_TCC_IN_V1) ? MIPS_R_V1 : MIPS_R_S4, -1);
+	tcc_reg = (ctx->flags & EBPF_TCC_IN_V1) ? MIPS_R_V1 : MIPS_R_S4;
+	emit_instr(ctx, daddiu, MIPS_R_T5, tcc_reg, -1);
 	b_off = b_imm(this_idx + 1, ctx);
-	emit_instr(ctx, bltz, MIPS_R_T5, b_off);
+	emit_instr(ctx, bltz, tcc_reg, b_off);
 	/*
 	 * prog = array->ptrs[index];
 	 * if (prog == NULL)
-- 
2.20.1